3.1

Hardware

The ED-WDM series is designed in a modular format using the industry standard 19” rack system. The complete version of the kit occupies two 3Ux84HP enclosures, the Optical Components Rack and the Electronics Rack, as shown in Figure 1. Each component part is contained in a standard 3Ux10HP cassette. This modular approach allows the system to be built up from the basics to more advanced levels.

Figure 1:

Complete ED-WDM educator kit and individual power meter module

The Optical Components rack is intended to house a range of passive components modules such as couplers, WDMs, an isolator, a circulator and fibre Bragg gratings. Patchcords are provided to allow the external connection between the module as required by the experimental procedures. An SC/APC style of connector is used predominately in the kit to allow easy reconfiguring of the optical set-ups and to minimise spurious backreflections.

The Electronics Rack is designed to house the lasers, power meters, photoreceivers and TEC controllers along with a variable optical attenuator and 50:50 coupler. This provides the instrumentation to allow the interrogation and investigation of the WDM components and systems. It is also fitted with a USB interface to allow PC control & monitoring of the instrument by the dedicated driver & display software supplied with the kit

The electronics modules are described below:-

3.2

Software

The software provided with the ED-WDM series has three parts:

3.3

Literature

As with all OptoSci kits the education objective is to provide a comprehensive package to the educator hence extensive literature support is provided. The literature pack for the ED-WDM series is split into three sections:

4.1

WDM Components

As mentioned above the ED-WDM: WDM Components kit is intended as the starting point of the suite. The kit contains a ITU grid DFB laser (λ_c≈1550nm), a pair of InGaAs power meters and a variety of standard optical components that are typically present in WDM systems. The components for characterisation include fused fibre couplers, a fuse fibre 1310/1550nm WDM, a micro-optic add-drop multiplexer (OADM) operating at the DFB wavelength, an isolator, a circulator and a fibre Bragg grating.

The kit provides students with the theory and practical ability to study the basics of optical component operation and characterisation. To achieve these objectives the following tasks are carried out:

4.1.2

Automated Measurements

If computer control is available the spectral behaviour of the DWDM components can also be examined using the λ-scan software program. This is suited to the study of the narrowband components i.e. the OADMs and the Bragg gratings. The set-up shown in Figure 2 allow both arms of the multiplexer to be studied simultaneously using λ-scan.

Figure 2:

Experimental set-up for spectral behaviour of the OADM

A typical results screen is shown in Figure 3(a), the students are directed save the data sets for further processing to normalise the plot and yield the actual insertion losses to each arm, as displayed in Figure 3(b).

Figure 3:

Spectral characterisation of OADM: (a) Screenshot from λ-scan and (b) Processed data

The software thus offers a simple way to acquiring the full spectral characteristic of the components, this is especially beneficial when dealing with the very narrow features associated with the fibre Bragg gratings as will been seen later.

4.1.3

DFB LVI plots

As one of the most important parts if the WDM systems, the students then carry on to investigate the operation of an DFB laser.

The DFB laser characteristics may be obtained using computer control and the LVI plotting software. Figure 4 shows a typical LVI plotter window from the software and the results of a series of L-I plots for a laser with variation of operating temperature.

Figure 4:

DFB laser characteristics: (a) LVI plotter window and (b) L-I plots against temperature.

As the operating temperature of the 1550nm laser is increased the threshold current is seen to increase and the slope efficiency decrease. Over this limited temperature range the results show the threshold current temperature dependence is 0.25mA/°C.

This should highlight to the students the marked temperature dependence of the laser characteristics thus making it imperative to have close control, not only for wavelength stability, but also to provide stable laser output power levels for DWDM systems.

4.2

1310/1550 WDM Systems

The first of the extension kits expands the fundamentals developed in the components characterisation kit to allow the investigation of practical WDM systems working at 1310/1550nm. The extension includes a second laser (λ_c=1310nm), dual photoreceivers and an additional fused fibre 1310/1550nm WDM and a reel of singlemode fibre. To study these WDM systems the students are directed to complete the tasks listed below:

4.2.1

Component characterisation at 1310nm

By repeating the component characterisation process at 1310nm the students will gain an insight into the broadband spectral behaviour of fibre optic components. In the kit a standard coupler and dual-window coupler are supplied to highlight different types that may be encountered, the dual window should show similar operation at both 1310nm and 1550nm whereas the standard type will operate only as specified at 1550nm. The micro-optic components (i.e. isolator and circulator) are specified at 1550nm hence the students should find operation at 1310nm to be outwith the expected values. Most importantly the availability of the second wavelength laser allows the complete characterisation of the fused fibre WDM as shown in Figure 5.

Figure 5:

1310/1550nm WDM definitions

From this series of measurements the students are asked to identify suitable connections to obtain the desired multiplexing and demultiplexing operations required for the WDM systems below.

4.2.2

1310/1550 WDM Systems

The basic characterisation of a simple WDM system shown in Figure 6 is carried out in a similar way to the single component by noting the power levels at either output from each input laser.

Figure 6:

Basic two-channel WDM system

In order to demonstrate the effects of multiplexing at each point in the system the lasers are then modulated with different signals. Photoreceivers are used to examine the output waveforms after multiplexing and demultiplexing on a suitable oscilloscope. The system and typical results are shown in Figure 7.

Figure 7:

WDM demonstration

4.2.3

Characteristics of Optical Fibre

The channel in any communications system comprises every element between the output of the transmitter and the input to the receiver. In an optical system the channel comprises the optical fibre cable plus a few connectors and/or splices, the receiver and transmitter interfaces (i.e. the terminations) and, in very long distance links, repeaters. The properties of the channel have a strong influence on the performance of the complete system. Digital signals transmitted by the channel are degraded by power loss (attenuation) and pulse spreading (dispersion) in the cabled optical fibre and additional losses are incurred at the splices, connectors and terminations. These effects in turn have a significant bearing on the maximum link length and bit rate.

OptoSci’s ED-COM, Fibre Optic Communications, and BER(COM) kits examine these effects in detail using LED and laser sources at wavelengths around 800nm and multimode fibre [3, 4]. With optical communications systems using 800nm sources and multimode fibre the attenuation and dispersion effects are larger than at 1310nm and 1550nm and, with appropriate educator kit design, enable dispersion measurements to be made with standard laboratory equipment. However, the general concepts demonstrated at 800nm are equally applicable to state of the art long haul, high capacity fibre links operating at 1310nm and 1550nm. In order to expand upon the experiments in the ED-COM and BER(COM) kits and investigate some of the characteristics of higher capacity fibre links, an experimental section was included in the ED-WDM: 1310/1550 WDM Systems extension examining some attenuation and dispersion phenomena at 1310nm and 1550nm.

The students start with a simple attenuation measurement of a fibre reel with a nominal length of 4.4km using the techniques described in the previous sections. An estimate of the fibre length is then made by applying an impulse modulation to the laser and measuring the time of flight. These measurements give typical attenuation co-efficients for the fibre of 0.22dB/km at 1550nm and 0.38dB/km at 1310nm – agreeing well with manufacturer specifications. This should emphases to the student that 1550nm is the lower-loss transmission wavelength.

The important topic of chromatic (intramodal) dispersion is then investigated by examining the transmission of the two wavelengths over various lengths of singlemode fibre. An elegant way of simulating different transmission lengths is by using a ring resonator arrangement as shown in Figure 8(a). The ring resonator is formed simply by connecting the coupled arm (Port 4) and second input (Port 2) of a 50/50 coupler with the fibre reel.

Figure 8:

Fibre chromatic dispersion measurements with ring resonator at 1310nm & 1550nm

Applying an impulse modulation to the lasers with this optical arrangement results in a series of pulses (of decreasing size) appearing at the receiver with time delays corresponding to integer multiples of the cavity length (nominally 0, 4.4, 8.8, 13.2, 17.6km) as is shown in Figure 8(b).

Taking a closer look at the output after each pass, as shown in Figure 9, demonstrates the effects of chromatic dispersion with the 1550nm pulse lagging the 1310nm by approximately 9.5ns every pass (4.453km). This illustrates the possibilities of pulse spreading caused by dispersion over long distances in optical fibre transmission channels.

Figure 9:

Increasing separation of 1310 & 1550 pulses after each pass round the ring resonator

4.3

DWDM Systems

With the ED-WDM: DWDM Systems extension students are expected to perform the investigation of practical DWDM systems. The following tasks are carried out:

The module provides a second DFB laser, operating at a channel adjacent to the original laser, dual InGaAs photoreceivers, a variable optical attenuator (VOA) and an additional OADM (at the adjacent channel wavelength).

In a manner similar to the 1310/1550nm WDM systems kit, this extension demonstrates the fundamental ability of the OADM components to efficiently multiplex and demultiplex signals onto a single optical fibre channel.

The system constructed by the students is shown in Figure 10 with the basic results identical to the oscilloscope trace shown in Figure 7.

Figure 10:

Two-channel DWDM system

The major difference from the 1310/1550 set-up being highlighted is the 0.8nm wavelength separation of the dense WDM channels. The adjacent nature of the channels used can then be used to demonstrate the effects of crosstalk on DWDM systems. Using the experimental set-up shown in Figure 10, the students are directed to detune the wavelength of one lasers from its centre wavelength towards the adjacent channel and examine the effect on the output waveforms. Figure 11 shows the increasing levels of crosstalk appearing on the 1549.32nm output port as the 1550.12nm laser is tuned to 1549.9nm.

Figure 11:

Demonstration of Crosstalk on a DWDM system

A second possible DWDM scenario is then examined. In some WDM systems, Channels may be added and dropped at various points along the optical link. Figure 12 shows a system where a second channel is added at some considerable distance from the Channel 1 input (simulated by high attenuation, ≈25dB, produced by a variable optical attenuator of the Channel 1 signal) and then the Channel 1 signal is dropped a short distance beyond that. The students are then asked to experimentally investigate this type of system in which a strong signal is present at the drop point for a weak signal. In particular they are asked to investigate the effects of wavelength drift in the Channel 2 laser source resulting in crosstalk.

Figure 12:

Crosstalk demonstration by adding a strong signal on λ² (modulated) to a weak signal on λ¹ (PRBS). The receiver is looking at the weak λ¹ signal with crosstalk from λ².

This arrangement can best be studied by using comparison of eye-diagrams and bit-error-rate (BER) analysis of the system operation as the laser wavelength is detuned. The OptoSci BER(COM) kit [4] provides a ready means to carry out this analysis and hence is recommended as a possible add-on. However any suitable PRBS generator may be used (directions for the use of external signal generators are provided in the technical appendices of the literature support).

The students should find that there is a noticeable increase in the noise levels present on the output trace with an associated closing of the eye. This is due to the OADM configuration where channels are more susceptible to crosstalk as the effective isolation is reduced due to the large difference in power levels from the new add channel relative to the low level of the original (attenuated) signal. Using the BER(COM) and its accompanying software to allows analysis of the eye-diagrams and estimated the BERs, a typical set of results (again provided in the instructors supplement) is presented in Table 1.

Table 1:

BER results for DWDM system.

The performance of the system can be seen to degrade and bit error rates increase rapidly in the configuration shown in Figure 12 and hence the control and accuracy of the laser operating wavelength becomes critical.

4.4

Bragg Gratings

The ED-WDM: Bragg Gratings extension concentrates on the topic of fibre Bragg Grating (FBG) Sensors investigating the effects of temperature and examining possible uses as temperature sensors. The kit includes a Bragg grating on a temperature controlled mount and a thermo-electric-cooler (TEC) module.

Figure 13:

FBG definitions

In order to perform the basic characterisation of a fibre Bragg the students are directed to use the experimental set-up shown in Figure 14. The insertion of the circulator is required to provide a measurement path for the reflected wavelength and to eliminate a return path to the laser source. Clear instruction is provided to ensure the resultant power levels are corrected for the additional power drops associated with passes through the circulator.

Figure 14:

Fibre Bragg grating characterisation

As mentioned earlier the response of the Bragg is very narrowband hence the use of the λ-scan software is recommended to achieve a full measurement set. A typical pair of responses for the transmit and reflect conditions is shown in Figure 15(a). Figure 15(b) shows how the transmission of the Bragg grating changes as the temperature is varied using the TEC module.

Figure 15:

Bragg Grating spectral responses, (a) transmission and reflection at 25 °C, and (b) transmission at 20°C, 25°C and 30°C

From Figure 15(b) the temperature co-efficient can be calculated as 20pm/°C. The changes in grating temperature trigger corresponding variations in the period of the grating and thus the wavelength of light that is reflected. This Bragg reflection shift makes it straightforward to use Bragg gratings to track variations in environmental parameters such as temperature and strain. The fact that multiple Bragg gratings can be written within an optical fibre also make these sensors amenable to direct and non-intrusive integration within the body of composite materials used in civil structures, aerospace platforms, etc. in order to provide detailed structural health monitoring information.